This invention is directed toward the measure of geophysical parameters of earth formations penetrated by a borehole. The invention employs propagation resistivity techniques spaced transmitters operating at different frequencies and a plurality of longitudinally spaced receivers. An electromagnetic wave is propagated from the transmitting antenna coil into the formation in the vicinity of the borehole and is detected as it pass the receiving antennas. The basic or "raw" parameters measured by the receivers are the phase and the amplitude of the passing wave. The downhole instrument is conveyed along the borehole making a plurality of raw measurements as a function of depth within the borehole from which geophysical parameters of interest are computed as a function of depth within the borehole. It is quite common in the prior art to first combine raw data measurement and then to compute parameters of interest from these process measurements. A typical example is the computation of apparent resistivity from the difference in phase of signals detected at receivers at different longitudinal spacings from the transmitter. A second example is the computation of apparent resistivity from the ratio of the amplitude of signals detected at the longitudinally space receivers. Such preprocessing or data combination is performed primarily to eliminate the gross effects of the borehole and is well known in the prior art. No attempt is made in the prior art to simultaneously track systematic errors associated with the measurements of apparent resistivity.
The current invention provides means and methods for determining error which can be related to uncertainty associated with geophysical parameters measured with a downhole instrument of the type previously described. The user of the information, or "analyst", selects the parameters of interest which might include the resistivity (or conductivity) of the formation, the dielectric constant of the formation, or perhaps the degree to which drilling fluids invade the formation in the vicinity of the borehole. The analyst's primary interests are usually the determination of the hydrocarbon saturation, porosity and permeability of the formations penetrated by the borehole. It is highly desirable to make such measurements while drilling or soon after the drilling of the well borehole so that critical economic concerning the amount and producibility of hydrocarbons in place can be made. Based upon this information, the well will either be completed or abandoned. The accuracy and precision of geophysical parameters selected to make such critical decisions is also of prime importance. The error measurements provided by the current invention can also be used to indicate equipment malfunctions of both the electrical and mechanical types. Although prior art teaches means and methods of measuring a wide range of geophysical parameters using electromagnetic techniques, little, if any, emphasis is placed upon determining the quality of the measurements. Usually the analyst can only rely on past experience in assigning, at best, qualitative estimates of the quality of the measurements obtained from the borehole instrument and associated system. Any error analysis is usually performed long after the measurements are made and usually not at the well site. Stated another way, prior art does not provide means and methods for determining the quality of electromagnetic based geophysical measurements in real-time, although real-time or near real-time economic and operational decisions are made based upon these measurements.
There is critical need for quantitative indications of the quality of geophysical measurements made in formations penetrated measurements simultaneous with the measurements made in formations penetrated by a borehole. More particularly there is a need for such quality measurements simultaneous with the measurements of parameters of interest. This is especially true in electromagnetic type measurements of formation resistivity which weighs so heavily in decision to complete or abandon the well. The present invention provides this very information by providing means and methods for measuring geophysical parameters selected by the analyst and simultaneously yielding quantitative measurements of the quality or error associated with the measurements of the selected parameters.